Implement zone-aware localities building logic for a locality weighted load balancing

*Description*:
I think it would be worth grouping endpoints by localities and specifying `load_balancing_weight` instead of route config's weighted_clusters. In case of zone aware routing this wasn’t possible because the locality weight isn’t considered.

In contrary to weighted clusters configuration this wouldn't break consistent hashing lb/ sticky sessions; hosts's health status would be considered and the weights properly adjusted in case of some endpoints getting unhealthy and so on.

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.